Could be very simple or very complex.
Is each Opp a separate transaction?
Are you able to record +1 transactions for each Opp or per Contact?
Are you asking about the count of tx or that Marketo influenced the Opps?
There are times when a transaction may never be associated to a CRM opportunity.
An example - We send an e-mail to a lead from Marketo about a solution and that prospect clicks on the offer link but does not purchase online or call their account executive (owner of opportunities) but instead they go to a retail store and purchase the solution. The retail store does not use a CRM do they scan the SKU into POS. That transaction goes to a data warehouse with no CRM opportunity associated to it. We are trying to create some connection to say that the program in Marketo drove that person to the retail store to purchase that solution.